1. Field of the Invention
The present invention concerns a hearing aid device of the type having an input transducer for acquisition of an input signal and conversion into an electrical signal, an A/D converter for conversion of the electrical signal into a digital signal, a signal processing unit for processing and amplification of the digital signal, a sigma-delta modulator for generation of at least one output bit stream, an output stage for generation of an electrical output signal and an output transducer for conversion of the electrical output signal into an output signal perceivable by a user.
2. Description of the Prior Art
In modern digital audio apparatuses, a component known as a sigma-delta modulator is used for conversion of digital signals into analog signals to activate a speaker or earphone. These sigma-delta converters transform the digital signal representation into a bit stream, which directly represents the acoustic output signal. Since the individual output bits of this output signal are output with a high rate, analog filtering typically must ensue for limitation to the required audio frequency range in order to keep the higher-frequency interference signals away from the speaker.
The speaker used in hearing aid devices, which speaker is typically called as earpieces and normally operates according the magnetic principle. Hearing device earpieces inherently exhibit a strong low-pass characteristic. In hearing aid devices with a sigma-delta modulator, the analog filtering of the output signal can be omitted. Due to the high system clock frequency of a sigma-delta modulator, its energy consumption is, however, quite high, which is disadvantageous for use in hearing aid devices. The argument against the selection of a lower (and thus more advantageous in terms of energy) system clock frequency is that the system noise would increase with such a lower frequency.
A hearing device is known from United States Patent Application Publication No. 2003/0081803 A1 in which a sigma-delta modulator generates an output bit stream with the three states +1, 0, −1. This bit stream is supplied to an output stage in the form of an H-bridge that delivers an output signal for direct activation of the earpiece. A circuit that initially, periodically converts the sigma-delta-modulated data stream from each value different from 0 to the 0-state is located between the sigma-delta modulator and the H-bridge. Overall energy is thereby taken from the output signal, so the system noise is also reduced. Disadvantages of this technique are that the non-linearities are generated as well as signal deformation.
A hearing aid device with a microphone, a transfer characteristic component for signal processing, and an output amplifier (which is essentially formed of a sigma-delta converter, a clock pulse generator and a low-pass filter) is known from EP 0 793 897 B1.
A sigma-delta modulator to which an FIR filter is connected is known from EP 0 815 651 B1.